1. Technical Field
The present invention relates to a surface light source device and a frame used therein. For example, the invention relates to a surface light source device that is used as a backlight illuminating a liquid crystal panel, and to a frame that surrounds a lightguide plate in the surface light source device.
2. Related Art
[First Conventional Example]
A liquid crystal display unit of a mobile phone is configured such that a surface light source device (backlight) for illumination is disposed behind a liquid crystal panel. FIG. 1 is an exploded perspective view illustrating a general structure of a surface light source device used in the liquid crystal display unit. FIG. 2A is a plan view of the assembled surface light source device, and FIG. 2B is an enlarged sectional view taken on a line X-X of FIG. 2A.
As illustrated in FIGS. 1, 2A, and 2B, in a surface light source device 11, a reflecting sheet 12 is bonded to a lower surface of a rectangular frame 13 made of a white resin, thereby covering the lower surface of the frame 13. Plural diffusion sheets and optical sheets 17 are accommodated in the frame 13 while being laminated on a surface (upper surface) on a light output side of a lightguide plate 14. Plural light sources 16 mounted on a lower surface of a flexible printed board 15 are also accommodated in the frame 13, and each light source 16 is disposed opposite an end face of the lightguide plate 14. A frame-shaped light shielding sheet 18 made of a black resin is bonded to the upper surface of the frame 13, whereby the edge of the lightguide plate 14 is covered with the light shielding sheet 18 while the lightguide plate 14 is retained between the light shielding sheet 18 and the reflecting sheet 12. For example, Japanese Unexamined Patent Publication No. 11-52140 discloses a surface light source device in which the frame made of the white resin is used. Japanese Unexamined Patent Publication No. 11-52140 also discloses a surface light source device in which an upper surface portion of the frame is molded using the black resin while a side surface portion of the frame is molded using the white resin.
Not only the frame 13 has a function of a case in which the lightguide plate 14 and the optical sheet 17 are accommodated, but also the frame 13 has a function of improving use efficiency of the light. As illustrated by a solid-line arrow in FIGS. 2A and 2B, when the light guided in the lightguide plate 14 leaks from a side surface of the lightguide plate 14, the leaked light is reflected by an inner surface of the frame 13 (sometimes titanium oxide is added to the white resin to enhance reflectance), and the light is incident again from the side surface of the lightguide plate 14. Therefore, the light that leaks from the lightguide plate 14 once can be utilized to improve the use efficiency of the light.
However, when the frame 13 made of the white resin is used, the light that leaks from the lightguide plate 14 is not completely reflected by the frame 13, but part of the light that leaks from the lightguide plate 14 is transmitted through the frame 13 to leak out as illustrated by a broken-line arrow in FIGS. 2A and 2B. Recently a display screen is increased in size while the liquid crystal display unit is thinned. For example, in a large-screen liquid crystal display apparatus provided with a touch panel, a width of the display screen is substantially equal to a width of the mobile phone. An area of an effective emission region is enlarged while external dimensions of the surface light source device is restricted. As a result, it is inevitable to narrow the frame of the surface light source device, and a thickness of the frame is thinned more and more. Consequently, an amount of light that is transmitted through the frame 13 to leak out is gradually increased.
When the light that is transmitted through the frame 13 to leak from the surface light source device 11 is increased, an outer peripheral surface of the surface light source device 11 is brightened to degrade visual quality of the liquid crystal display unit. When a photosensor or a light receiving portion for infrared communication is disposed near the surface light source device 11 incorporated in the mobile phone, the light that leaks from the outer peripheral surface of the surface light source device 11 possibly causes a malfunction of the photosensor or the light receiving portion.
[Second Conventional Example]
As described above, Japanese Unexamined Patent Publication No. 11-52140 discloses a surface light source device in which the frame is molded by the black resin. In the surface light source device in which the frame is molded by the black resin, because the light that leaks from the side surface of the lightguide plate is absorbed by the side surface portion of the frame made of the black resin, the light hardly leaks out from the side surface of the surface light source device.
However, in the surface light source device in which the frame is molded by the black resin, the use efficiency of the light of the surface light source device is degraded because the light that leaks from the side surface of the lightguide plate cannot be reflected by the side surface portion of the frame and incident to the lightguide plate again. As a result, the amount of light output from the effective emission region of the surface light source device is decreased to degrade emission luminance.
As a result of an experiment and a simulation, which are performed to demonstrate the degradation of the emission luminance, the following results were obtained. In the surface light source device in which the frame having four sides made of the white resin was used, it was assumed that the average luminance was set to 100% when the luminance was measured at 25 points (the number of measuring points) in the emission surface. In the surface light source device that has the same structure except that the frame having four sides made of the black resin was used, the average luminance of 92 to 93% was obtained when the luminance was measured at 25 points. In the surface light source device in which the frame having four sides made of the white resin was used, it was assumed that the average luminance was set to 100% when the luminance was measured at 289 points in the emission surface. In the surface light source device that has the same structure except that the frame having four sides made of the black resin was used, the average luminance of 90 to 92% was obtained when the luminance was measured at 289 points. In the surface light source device in which the frame having four sides made of the white resin was used, it was assumed that the average luminance was set to 100% when the luminance was simulated at 25 points in the emission surface. In the surface light source device that has the same structure except that the frame having four sides made of the black resin was used, the average luminance of 90% was obtained when the luminance was simulated at 25 points. Therefore, when the black-resin frame is used, it is found that the black-resin frame absorbs about 10% of the mount of light output from the emission surface in the surface light source device in which the white-resin frame was used.